This invention relates to sailing vehicles such as boardsailing craft, and more particularly to the form and structure of the interface between a sail and a mast.
Conventional boardsailing technology is illustrated in FIG. 1 showing the leading edge of a sail wrapped around a cylindrical mast and attached thereto by a seam forming a pocket or sock for the mast. The seam forms a concave discontinuity on each side of the sail to the rear of the mast. These discontinuities induce turbulence which reduces the efficiency of the sail by increasing drag and decreasing lift generated by the sail. "Lift" herein means a force normal to both relative wind and the mast for propelling the vehicle produced by wind passing the sail leading edge proximate to the mast toward the sail trailing edge. "Drag" wherein means a force normal to the lift and to the mast caused by resistance to relative wind passage over the mast and sail. Additional drag is produced with conventional technology because of the non-optimum cylindrical shape of the leading edge.
In this specification the terms "front", "top" and "head" refer respectively to upwind, lift and away from the vehicle directions. The directions "rear", "bottom" and "foot" are opposite to the respective "front", "top" and "head" directions respectively.
An improvement in the prior art is the use of an airfoil shaped mast as shown in FIG. 2. The front edge of the sail is supported by a bead engaging a slot in the rear of the mast. This design provides an efficient shape at the front of the mast, a relatively thin trailing edge at the rear of the mast and, when properly trimmed, a nearly continuously smooth shape on the top (low pressure) side of the sail at the rear of the mast.
The main disadvantage of the airfoil shaped mast is that the concave discontinuity on the bottom (high pressure) side is even greater than that formed by the conventional attachment described above when the curvature of the sail matches that of the rear top side of the mast.
Another disadvantage of the airfoil shaped mast is that turbulence will be generated on the top side unless the sail is trimmed to match the curvature of the top side of the mast at the point of attachment. Proper trim is illustrated in FIG. 2 by position (a) of the sail. In this position the curvature of the sail matches the curvature of the rear top side of the mast. When variant relative wind direction and/or velocity conditions are encountered, it may be desirable for overall sail performance to trim the sail to either position (b) or position (c) as shown in FIG. 2. The conditions illustated by positions (b) or (c) produce the disadvantage of increased drag caused by turbulence at the rear of the mast due to a convex (position (b)) or concave (position (c)) discontinuity on the top side.
Another disadvantage of the airfoil shaped mast is that the conditions illustrated by positions (b) and (c) in FIG. 2 produce additional drag at the leading edge of the mast because the mast leading edge does not face properly into the relative wind.
To correct for the discontinuity at the top side between the airfoil shaped mast and the sail, and to align the leading edge into the relative wind, the prior art adds a further adjustment, that of rotating the mast. This added adjustment is costly to produce, subject to mechanical failure, and burdens a sailor with an additional function to monitor.
A further disadvantage of the prior art mast rotational adjustment is that the optimum position for matching sail curvature will not always position the leading edge directly into the relative wind.
In view of these problems, there is a need for a sail attachment to a mast that provides an efficiently shaped, properly oriented, leading edge of the sail without introducing turbulence on either the top or bottom sides of the sail, and that is automatic in operation.